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Our field team arrived at the Copacabana Field Station in Admiralty Bay, King George Island, on 6 October 1995, to continue the long-term ecological and demographic studies of the region's penguin and flying bird populations. Brash ice lined the beach and fast ice filled the inlets, while the center of the bay was ice free. Polish scientists at nearby Arctowski Station reported that pack ice had covered the bay all winter and had moved out only a few days before our arrival (members of XX Polish Expedition personal communication). The snow had melted from the penguin breeding areas and both Adélie and gentoo penguins were attending pebble nests.
Our research had four main objectives:
We monitored penguin reproductive success and demography by following all known-aged birds plus a random sample of 150 nests each of Adélie and gentoo penguins throughout the breeding season. For the reproductive study, 100 penguins of each study species were banded, and the remaining 50 pairs served as a study control. Daily observations indicated that both the Adélie and gentoo penguins began egg laying in the third week of October. Egg-laying synchroneity was high for both species and peak egg laying occurred between 27 and 31 October. In comparison to 1994, these peak dates were a week earlier for Adélie penguins and two weeks earlier for gentoo penguins. The advanced, highly synchronous egg-laying dates were most likely prompted by the early availability of snow-free nesting ground. Adélie penguin first incubation shifts averaged 11 days for males and 8 days for females, similar to previous years' data. Breeding success was high, with both species fledging more than one chick per pair. Population censuses indicated that the number of Adélie penguins breeding at the Copacabana colony increased slightly, whereas gentoo and chinstrap penguin numbers declined slightly from 1994. The large number of banded, known-age Adélie penguins sighted throughout the season indicated that recruitment of young Adélie penguins was high.
Chinstrap penguins arrived late in October, and egg laying occurred in the third week of November, consistent with previous years. Chinstrap penguins also fledged approximately one chick per pair, indicating that the 1995-1996 season was a productive breeding year for all three Pygoscelis species.
Brown skuas arrived and began setting up territories in early November, followed by south polar skuas approximately 2 weeks later. Reproductive success was slightly higher for brown skuas than for south polar skuas, though they both averaged one fledged chick per pair. South polar skua diets were sampled through guano collection at their nesting territories. Analysis of fish otoliths in the guano during courtship, egg laying, and chick rearing stages indicates that Electrona antarctica and Pleurogramma antarcticum were the dominant fish prey. Krill (Euphausia superba) was the other dominant prey item. We also monitored the reproductive success and demography of southern giant petrels, kelp gulls, and American sheathbills. No American sheathbills bred successfully near the field site this season; however, kelp gull and southern giant petrel nest numbers remained consistent with 1994. Kelp gulls fledged less than two chicks per pair and giant petrel reproductive success remained high, averaging 0.75 chicks fledged per pair.
Our second study objective was to quantify the diets of the three Pygoscelis penguin species, during the chick-rearing stage, using the water off-loading technique (Wilson 1984). Independent samples of five penguins per species per week were lavaged over 6 weeks to determine the weights and contents of the stomachs. Only penguins with active nest sites containing chicks were sampled. E. superba was the dominant prey item found in all three of the penguin diets. A random sample of 50 E. superba from each stomach were measured and sexed. A length-frequency analysis of E. superba size depicted a binomial distribution of both smaller/younger (16-30-millimeter) and larger/older (46-65-millimeter) animals. These data indicate the abundance of both young and old cohorts in the penguin diets and support the krill super cohort hypothesis (Trivelpiece and Trivelpiece in press) predicted for 1995-1996.
Our third objective at Copacabana Field Station was to continue the Adélie penguin weight study by monitoring weight fluctuations in breeding Adélie penguins throughout the season. The penguins used were those that had been banded and followed for the previous year's reproduction study. Each penguin was monitored daily to record dates and weights for arrival; egg laying; incubation shifts; chick hatching; and first, second, and third week of chick rearing. This study provides data on arrival weights, weight gain/loss, and incubation shifts that can be used to index foraging success and food availability throughout the season and over the years.
Our final objective was to monitor leopard seal behavior and predation events at the Copacabana penguin colony. We wanted to identify both biotic and physical variables that might influence leopard seal presence, activity, and foraging success. Observations were made from a blind 15 meters above Admiralty Bay, 50 meters from the beach. Leopard seal and penguin activity was recorded during 3-hour observation periods that rotated sequentially from dawn till dusk throughout the austral summer. Atmospheric and oceanic conditions were measured and continuous searches were made for leopard seals and penguin predation events. The number of leopard seal sightings and predation events were low during the 1995-1996 summer. Preliminary analysis indicates that leopard seal activity was more strongly correlated to the presence of pack ice in Admiralty Bay than to penguin activity at the Copacabana colony.
Whale sightings collected opportunistically throughout the season indicated that the numbers of whales sighted in Admiralty Bay were below the 10-year mean for orca (Orcinus orca), humpback (Balaenoptera novaeangliae), and minke (Balaenoptera acutorostrata) whales. Weddell (Leptonychotes weddellii) and crabeater (Lobodon carcinophagus) seals were present throughout the season, though not abundant. Fur seals (Arctocephalus gazella), which typically arrive around mid-January in large numbers, were delayed until early February. A number of flying bird vagrants also occurred in Admiralty Bay this season including yellow-billed pintail (Anas georgica), white-rumped sandpiper (Calidris fuscicollis), pectoral sandpiper (C. melanotos), and black-necked swan (Cignus melancoryphus).
We would like to express special appreciation to the crew of the R/V Polar Duke and the U.S. Antarctic Program for logistical support and the members of the XIX and XX Polish Antarctic Expeditions. This study was funded by National Science Foundation grant OPP 91-21952.
References
Trivelpiece, W.Z., and S.G. Trivelpiece. In press. The impact of global warming on Antarctica's krill-based food web and implications for fisheries management in the southern ocean. Ecology.
Wilson R.P. 1984. An improved stomach pump for penguins and other seabirds. Journal of Field Ornithology, 55(1), 109-112.
XX Polish Expedition. 1995. Personal communication.